Many applications, especially in the field of optics, require the use of high purity fused silica (HPFS®) articles having specific shapes. An example of such articles is a lens. The size of such articles may vary from very small to very large. The formation of such articles from the HPFS® material is a challenge in the art, especially for those having a large size and/or complex shape.
The HPFS® material in many applications, such as reticles and stepper lenses in modern photolithography for the production of VLI, is required very good transmission properties at short wavelength such as 248 nm, 193 nm and even shorter. In addition, the lens materials are often required of sufficient durability against excimer laser irradiation. These stringent property requirements call for tightly controlled composition of the glass.
The “direct method” for manufacturing HPFS® glass ingot typically involves the following general steps: A combustion-supporting gas, such as oxygen, and a combustible fuel gas (hydrogen, CH4 or natural gas) are mixed and burned by a burner made of a synthetic silica glass. A high purity silicon containing precursor material, such as silicon chloride, silane or silicon octamethyltetrasiloxane (OMCTS) is diluted with a carrier gas, such as O2, H2, inert gas, etc., and is emitted from the burner. Fine particles of a synthetic silica glass is produced by causing the precursor feedstock to react with the water (hydrolysis reaction) produced by the combustion of the surrounding oxygen gas and the fuel gas. These fine particles of synthetic silica glass are deposited on the surface of a target made of an opaque synthetic silica glass plate located beneath the burner. During the deposition, the target is subject to rotation, oscillation and downward-pulling motions. These fine particles are simultaneously melted and vitrified by the heat of the combustion of the fuel gas. If this method is used, a synthetic silica glass ingot with a relatively large diameter can be obtained.
Another method for manufacturing synthetic silica glass ingots is a so-called VAD (vapor-phase axial deposition) method, which utilizes techniques used in the manufacture of optical fibers. In this method, a porous synthetic silica glass rod is made and then consolidated by performing a heat treatment in an atmosphere, such as Cl2 and/or He/O2. Cl2 treatment is usually used to reduce the OH level in the glass.
The synthetic silica glass ingots manufactured by these methods are cut to form glass blocks (synthetic silica glass bulks) that have a desired shape and size to be used as optical members, such as lenses, etc. However, the blocks of glasses oftentimes do not have a size large enough for the intended applications. For example, with respect to many silica glass articles requiring very low OH level, the originally supplied glass may not have the diameter for direct mechanical processing, such as machining and grinding. Thus, a small diameter billet of silica glass with the same volume has to be reflowed into a larger diameter.
The current processes for the reflow of low OH specialty HPFS® material are very complex. This is because the reflow process must not alter the glass composition in a detrimental manner. For example, care should be taken not to introduce O2 molecules and/or OH into the glass during the process. This is especially difficult because the high temperature required for the reflow is oftentimes provided by oxygen burner. A current utilized process involves placing the low OH glass sandwiched between two layers of protective glass (cap glass), which could have higher OH level. The heated sandwich structure is then allowed to sag over a form so as to make a semispherical shape or other shape for further processing. This process uses oxygen fires for the addition of heat for the sagging. The cap glass protects the low OH glass from O2, OH and/or other contamination during the process along with volumetric hydrostatics. This process involves secondary operations for the removal of the cap glass and then finishing the low OH glass to the final specifications.
Therefore, a direct mold casting process without the need to use the cap glass layers would be advantageous, as long as the process does not alter the glass composition in a detrimental manner.
However, casting near net-shape silica articles directly without introducing property-altering contaminants is not without difficulty. First, a very high processing temperature is required for the silica material to reach a sufficiently low viscosity. This poses great challenges to the mold design. Second, the cast near net-shape article shall have a configuration close to the end product, so that minimal post-casting finishing is required. This requires the molten glass to flow to fill the mold cavity without leaving gas pockets. Third, the cast articles shall be substantially free of defects, such as bubbles. These requirements are not easy to satisfy.
An example of the synthetic silica glass molding method is disclosed in Japanese Patent Application Kokai No. S56-129621. In that method, the silica glass bulk is molded by heat and pressure in a graphite molding vessel in a helium gas atmosphere with an absolute pressure between 0.1 Torr and the atmospheric pressure at a temperature of 1700° C. or higher. Thereafter, the molded glass is rapidly cooled to a temperature between 1100° C. and 1300° C. Furthermore, a molding method in which the graphite molding vessel has an upright structure split into two or more sections is disclosed in Japanese Patent Application Kokai No. S57-6703. A method in which molding is performed at 1600° C. to 1700° C. using a graphite molding vessel having a structure that relaxes stresses caused by the differing thermal expansion coefficients between the synthetic silica glass and the molding vessel is disclosed in Japanese Patent Application Kokuku No. H4-54626.
These processes suffer from various drawbacks. Gas bubbles are generated in the synthetic silica glass bulk during the pressing-molding at high temperatures, and these gas bubbles remain in the synthetic silica glass in large quantities after the molding. Synthetic silica glass containing such large quantities of residual gas bubbles cannot be used as optical members. In addition, at high temperatures, the synthetic silica glass bulk may react with the constituent materials of the molding vessel. For example, where graphite is used as the molding vessel, the following reaction may take place:

Because of the formation of the CO gas, undesirable recesses and projections may be formed on the surface of the glass article. As a result, near net-shape glass articles are difficult to form according to the teachings of methods in these references. Further, as a result the pressing treatment in a graphite molding vessel, the optical characteristics of the resulting synthetic silica glass member (especially the uniformity of the in-plane transmission) are often degraded. If a synthetic HPFS® glass article having such an insufficient uniformity in the in-plane transmission is incorporated in an exposure apparatus as a member, the image-focusing performance of the exposure apparatus significantly degrades, which is undesirable.
U.S. patent application Publication No. 2002/0050152 A1 discloses a method for molding a synthetic silica glass member. The method includes accommodating a synthetic silica glass bulk inside a molding vessel; interposing an elastic member having a ventilating property between a pressing member and the synthetic silica glass bulk; providing a fastener for fastening at least peripheral edge portions of the elastic member to the pressing member; and pressing the synthetic silica glass bulk against the molding vessel by the pressing member in a high-temperature condition to mold the synthetic silica glass bulk into a synthetic silica glass member having a shape conforming to a shape of the space defined by the pressing member and the molding vessel, the synthetic silica glass bulk being pressed in such a manner that the pressing member and the elastic member tightly fasten to each other through the fastener. The molding apparatus as disclosed in this reference includes a molding vessel configured to accommodate a synthetic silica glass bulk; a heater for heating the molding vessel; a pressing member that presses the synthetic silica glass bulk in a high-temperature condition against the molding vessel to mold the synthetic silica glass bulk into a synthetic silica glass member having a shape confirming to the shape of a space defined by the pressing member and the molding vessel. According to the teaching of this reference, the fused silica material is molded into the desired shape by pressing. Therefore, the glass article thus produced may have the same in-plane transmission uniformity problem as mentioned above associated with other methods disclosed in other references discussed supra. In addition, it is clear that the molds and molding process as taught in this reference cannot be used for producing articles have a complex shape. Furthermore, from the description of the mold in this reference, it is known that large amounts of elastic materials made of carbon fiber or ceramic fibers, or other refractory materials are used. These elastic members are subject to substantial tensile stress and compressive stress during the molding process, due to the mechanical force exerted by the pressing member, as well as the coefficient of thermal expansion mismatch between the molding vessel and silica glass. This calls for considerable consumption of highly refractory materials. These elastic members all have large direct contact area with the molten silica during the molding process. Migration of contaminants from the elastic member to the glass article during the casting process is a big issue for this mold. Therefore, for applications that require very high purity of the silica article, the requirements as to the purity and composition of the elastic materials are very stringent. All these factors add to the complexity and cost of the mold.
Therefore, there is a genuine need of a high temperature mold and mold casting process for the production of near net-shape articles, such as fused silica articles having complex shapes.
The present invention satisfies this need.